In disk drives, defects on the media surface can cause the read channel to repeatedly detect incorrect data (hard errors). Very large defects may result in hard errors that are too long for the disk drive ECC algorithm to detect. Defect scans are used in the manufacturing process to flag those sectors with large defects so they are excluded from use during normal drive operation. The defect scan involves two main steps:
1. Write entire media surface with a high frequency repeating patterns;
2. Read back each sector and check for unusual changes in head signal amplitude.
A high frequency pattern is written to maximize the probability of is actually writing a transition on a small defect. The highest frequency pattern a read channel can usually write is a 2T preamble (transitions are spaced 2×TTBG apart). If a transition is written on a defect, the resulting magnetic head amplitude increases or decreases based on the type of defect. A decrease in magnetic material on the media correspondingly decreases the amplitude of the read back signal (resulting in a localized read back signal drop-out) and an increase in magnetic material on the media correspondingly increases the amplitude of the read back signal (resulting in a localized read back signal drop-in).
Based upon the number of detected defects, it may be determined whether the disk drive is useable or not. The disk drive may fail the manufacturing process when too many defects are detected. Conventionally, if the disk drive is determined to be usable given the number and severity of the detected defects, a predetermined space around the detected defect is designated as a margin that becomes unavailable for user data. However, experience has shown that a detected defect may spread or “grow” from its original position during subsequent use of the disk drive. Such defects are commonly known as “grown defects” and often manifest themselves outside of the predetermined margins designated around the detected media defect. In some cases, the disk drive may ultimately fail in the hands of the end user, due to such grown defects. To provide increased drive reliability, there is a need in the disk drive manufacturing process to enable better defect margining around detected media defects.